The present invention relates to a method and an apparatus for controlling a speed ratio of a continuously variable transmission for a vehicle wherein the output of an engine is transmitted to drive wheels with stepless or continuously variable speed ratios.
In the art of a continuously variable transmission for a vehicle to transmit a rotary motion of an engine to drive wheels with continuously variable ratios, it is common to change a speed ratio of the variable transmission for attaining a minimum specific fuel consumption. For this purpose, for example, a required horsepower of the engine is first obtained as a function of an amount of depression of an accelerator pedal or as a function of an angle of opening of a throttle valve. Then, a desired or target engine speed contemplated by a vehicle driver, corresponding to the operating amount of the accelerator pedal or the opening angle of the throttle valve, is determined based on a relation predetermined so as to provide the required horsepower of the engine with the minimum specific fuel consumption. The variable transmission is controlled such that an actual engine speed coincides with the determined target engine speed. Such a known method or an apparatus to practice this method is satisfactory while the vehicle is running in a relatively stable condition or at a relatively constant speed. However, in usual driving conditions wherein the accelerator pedal is frequently operated, the operating amount of the pedal or the opening angle of the throttle valve does not necessarily faithfully reflect the actual horsepower or current output of the engine. In other words, the known method or apparatus is disadvantageous in its capability of accurately establishing the target engine speed while the engine operation is in a transient state. In the same known arrangement, it is further noted that a slight variation in the opening angle of the throttle valve or in the operating amount of the accelerator pedal will cause a comparatively larger variation in the amount of supply an air-fuel mixture to the engine at relatively low engine speeds than at relatively high engine speeds, and consequently causes a comparatively larger variation in the engine output at the lower engine speeds. This fact makes it comparatively difficult to establish the target engine speed for optimal specific fuel consumption. Further, it is generally recognized that the transmission efficiency of a continuously variable transmission is reduced as its speed ratio is increased. Thus, the traditional continuously variable transmission is not completely satisfactory in its efficiency, and the target engine speed may not be determined exactly at an optimum point which must be determined with the efficiency characteristics of the variable transmission taken into consideration.